Definition and Measurement of Pressure: Atmospheric Pressure, Barometer, and Manometer

Recall that a gas always assumes the shape and volume of the vessel in which it is contained. Microscopically, a gas consists of moving particles that collide with each other and the walls of their container.

Each collision with the wall exerts a small force on the area of contact. The forces resulting from a large number of such collisions add up to its pressure.

Fewer gas particles lead to fewer collisions, a lower force per unit area, and hence, a lower pressure. Pressure, therefore, is directly proportional to the number of particles in a given volume, or the density of the gas.

Air — a mixture of different gaseous atoms and molecules — exerts pressure on everything on the surface of the earth. This is atmospheric pressure, and it is measured using a barometer. A traditional barometer is a mercury-filled glass tube that is inverted into a dish containing mercury.

In response to pressure exerted by the atmosphere, the height of the mercury column rises or falls. Measuring the height provides a measure of the atmospheric pressure.

At sea level, the atmospheric pressure is 1 atm, which raises the column of mercury to a height of 760 millimeters of mercury. At higher altitudes, whe